The invention concerns a method for the manufacture of the cylindrical mantle of the rolls or cylinders of a paper machine out of a corrosion-proof metal or alloy material, particularly of refined steel, and particularly for the manufacture of the cylindrical mantle to be perforated for a suction roll of a paper machine.
The invention also concerns a roll or cylinder of a paper machine, in particular a suction roll provided with a perforated mantle, comprising a cylindrical mantle and end pieces attached to both of its ends, in connection with which end pieces there are the axle journals of the roll or cylinder.
Rolls of paper machines, in particular suction rolls, operate in an environment which is highly corrosive. Moreover, suction rolls, in particular press rolls, are subjected to high dynamic loads, because at present the linear loads employed, e.g., in press rolls are of an order of 70-120 kN/m or even higher. This is why, in the mantles of suction rolls or the equivalent, it is necessary to use extensively alloyed special steels, such as two-phase, i.e. so-called duplex steels, which are expensive and difficult to work when cold. Problems of strength in the case of the mantles of suction rolls are also caused by the fact that their mantles are perforated, one mantle comprising typically about 500,000 suction holes.
The diameters of prior-art suction rolls are, as a rule, of an order of 600 . . . 1400 mm, and their wall thicknesses are 55-90 mm, in the case of large paper machines usually 70-90 mm. The lengths of the suction rolls correspond to the width of the paper machine, being usually within a range of 5-10 m.
The cylindrical mantles of suction rolls or equivalent for paper machines are, in prior art, manufactured by means of the following techniques. The cylindrical mantles are bent by being rolled out of a plate almost to the shape of a full circle or a semicircle, and the longitudinal joint or joints is/are welded together. Correspondingly, it is known in the prior art to bend cylindrical mantles from a plate to a curved shape by chamfering to a semicircular shape, whereupon the longitudinal joints of the cylinder halves are welded together. Chamfering can, as a rule, be employed up to a mantle wall thickness of about 50-70 mm only. Hereupon the cylindrical mantle made by rolling or chamfering is machined to cylindrical shape.
In the prior art, cylindrical mantles for paper machine rolls or cylinders are also manufactured by means of centrifugal casting. In this casting process the casting mould is made to revolve in a horizontal position, e.g., on rolls, and molten metal is fed into the mould, said metal remaining and solidifying on the mould walls by the effect of centrifugal forces.
Since the wall thicknesses of suction rolls and equivalent are quite high (55-90 mm), the rolling and chamfering of the plate material requires particularly robust equipment and high forces. In spite of this, for example when rolling is used, the roll mantle must be composed of axial parts of a length of about 2-3 m. With larger mantle thicknesses, higher than about 70 mm, rolling is not possible except by means of particularly robust equipment or by using very short mantle portions which are hot. Thus, with higher mantle thicknesses, it has been necessary to use chamfering, by which means it is, however, difficult to bring the mantle to precisely circular shape, which results in the drawback that large quantities of material must be machined off the mantle. This increases the time taken by the machining and the loss of material.
In a mantle manufactured by rolling or chamfering, it has been necessary to use transverse welding joints, which has resulted in the following drawbacks. In practice it has been noticed that the major part of the suction rolls are broken down by breaking off so that mostly the breaking point is exactly at the location of a transverse weld when the roll has been manufactured by welding. The points of starting and ending of a transverse weld are particularly risky problem points. This is why one of the main objectives of the present invention is to provide a process for the manufacture of a roll mantle and a roll or cylinder manufactured by means of said method wherein there are no transverse joints at all.
It is a drawback in the use of centrifugal casting methods that about one half of the wall thickness must be machined before a "sound" roll mantle is obtained. It is a further drawback that casting flaws tend to remain in the roll mantle, said flaws constituting starting points for fractures. A considerable drawback is the above proportion of material lost on machining, because the extensively alloyed steels used in roll mantles are very expensive.